Validate target status for VuMark instance generation

[adamtheturtle]

Summary

• Return TargetStatusNotSuccessError when a VuMark instance generation request (POST /targets/<target-id>/instances) targets a VuMark target not yet in the success state
• Add processing_time_seconds, last_modified_date, upload_date fields and a status property to VuMarkTarget, following the same pattern as ImageTarget
• Add get_vumark_target method to VuMarkDatabase for target lookup by ID
• Add tests for the processing target error, raw HTTP response, successful target, and VuMarkTarget/VuMarkDatabase serialization round-trips

Closes #2981

Test plan

• All 478 tests pass (5 new tests added)
• All pre-commit hooks pass (including mypy, pyright, ruff, vulture, custom linters)
• test_processing_target - verifies TargetStatusNotSuccessError is raised for a processing VuMark target
• test_processing_target_raw_response - verifies HTTP 403 with correct result code
• test_successful_target - verifies a ready target generates a VuMark instance successfully
• test_vumark_target_to_dict - verifies VuMarkTarget serialization round-trip
• test_vumark_database_to_dict - verifies VuMarkDatabase serialization round-trip

---

Note

Medium Risk
Changes request-handling behavior for a public-facing mock API endpoint and adds new target state/timestamp serialization, which could affect existing tests/clients relying on previous always-success behavior.

Overview
VuMark instance generation (POST /targets/<id>/instances) now validates the target’s status and returns TargetStatusNotSuccessError (HTTP 403) when the VuMark target is still processing (implemented in both the Flask server and the requests-mock server paths).

VuMarkTarget is extended to behave more like ImageTarget, adding processing/timestamp fields plus a computed status, and VuMarkDatabase gains get_vumark_target for ID-based lookup. Tests cover the new forbidden error behavior (including raw HTTP response), successful generation once processed, and VuMark target/database serialization round-trips.

^{Written by Cursor Bugbot for commit 62501fa. This will update automatically on new commits. Configure here.}